The present invention relates to a variable displacement type compressor for use in vehicle air-conditioners. More particularly, this invention relates to an apparatus for controlling the discharge capacity of a variable displacement type compressor by changing the tilt angle of a cam plate with a control valve.
This type of control apparatus known has a control passage that connects a discharge pressure area to the crank chamber, which houses a cam plate, and adjusts the difference between the pressure in the crank chamber and the pressure in the cylinder bores to change the tilt angle of the cam plate, thereby adjusting the discharge capacity. The adjustment of the difference between the pressure in the crank chamber and the pressure in the cylinder bores is carried out by changing the position of the displacement control valve, which is located in the control passage, under the control of a computer.
Japanese Unexamined Patent Publication (KOKAI) No. Hei 6-341378 discloses a displacement control valve that has a constant differential pressure valve section and an electric driving section. The displacement control valve performs control such that the difference between the pressure of the intake refrigerant gas (hereinafter referred to as the suction pressure), which has a correlation with the pressure in the cylinder bore, and the pressure in the crank chamber becomes equal to a preset value. More specifically, the constant differential pressure valve section adjusts the restriction of the control passage by actuating the valve body to keep the difference between the pressure in the crank chamber and the suction pressure at the preset value. An electric driving section changes a reference set value for the operation of this valve section by adjusting the load acting on the valve body under the control of the computer.
When the suction pressure rises to make the difference between the pressure in the crank chamber and the suction pressure fall below the set value, the valve section actuates the valve body to open the control passage. This increases the amount of the high-pressure refrigerant gas supplied to the crank chamber from the discharge pressure area, thus raising the pressure in the crank chamber. As a result, the difference between the pressure in the crank chamber and the suction pressure is maintained at the preset value.
When the suction pressure falls, making the difference between the pressure in the crank chamber and the suction pressure greater than the set value, on the other hand, the valve section moves the valve body in the direction to close the control passage. This decreases the amount of the high-pressure refrigerant gas supplied to the crank chamber from the discharge pressure area, thus dropping the pressure in the crank chamber. This keeps the difference between the pressure in the crank chamber and the suction pressure at the set value.
The computer compares the temperature detected by a temperature sensor with the temperature set by a temperature setting unit to determine a target value, and controls the electric driving section in such a way that the reference set value for driving the valve section is the target value.
When the cooling load acting on the compressor is heavy, for example, the difference between the temperature detected by the temperature sensor and the temperature set by the temperature setting unit is larger. Based on this large difference, the computer controls the electric driving section to decrease the reference set value for driving the valve section. As a result, the tilt angle of the cam plate increases based on a small difference between the pressure in the crank chamber and the pressure in the cylinder bore, thus increasing the discharge capacity of the compressor in accordance with the heavy cooling load.
When a light cooling load is acting on the compressor, on the other hand, the difference between the temperature detected by the temperature sensor and the temperature set by the temperature setting unit is smaller. Based on this small difference, the computer controls the electric driving section to increase the reference set value for driving the valve section. As a result, the cam plate decreases the tilt angle based on a large difference between the pressure in the crank chamber and the pressure in the cylinder bore via the associated piston, thus reducing the discharge capacity of the compressor in accordance with the light cooling load.
In the above-described compressor, however, moment M in the direction of increasing the tilt angle is acting on a cam plate 102 based on inertial force F of a piston 101 which reciprocates, as shown in FIG. 4. That is, in addition to the difference between the pressure in the crank chamber and the pressure in the cylinder bore via the piston 101, the moment M that acts on the cam plate 102 based on inertial force F of the piston 101 greatly affects the determination of the tilt angle of the cam plate 102. The magnitude of moment M is not always constant. As the rotational speed of the engine increases, the rotational speed of a drive shaft 103 rises too. When the piston 101 reciprocates at a high speed accordingly, the inertial force F of the piston 101 that acts on the cam plate 102 increases, thus making the moment M greater.
FIG. 5 shows a state where the cooling load is constant and the set value for driving the displacement control valve is also constant. In other words, FIG. 5 shows the state where the difference between the pressure in the crank chamber and the pressure in the inertial force F of the piston 101 cylinder bore via the piston 101 is maintained at a certain value. Even in such state, an increase in the rotational speed of the engine driving the compressor and an increase in the inertial force F of the piston 101 will increase the tilt angle of the cam plate 102 or the discharge capacity of the compressor. Such a low-precision control on the discharge capacity of a compressor without considering variation in the inertial force F of the piston 101 is apt to degrade the cooling performance of the air-conditioning system of a vehicle.